Early South American grasses (Paleocene-Eocene) occupied forested ecosystems

STILES, E.; STRÖMBERG, C.; CANARES, B. A.; GELFO, J. N.,; KOHN, M.; ERRA,G. ; GOIN, F. J.; MADDEN, R.; TRAYLER, R.; BAUZÁ, N.

Congreso; Botanical Society of America annual meeting; 2023

Grasses (Poaceae) exhibit exceptional taxonomic and ecological diversity, with grassland-dominatedhabitats around the world covering about 40% of Earth’s land surface. Modern grasslands occur intemperate to tropical ecosystems spanning low to high altitudes, a wide range of temperatures, andwet to arid climates. However, the ecology of early grasses and their environmental affinities remainpoorly understood because of the scant fossil record of grasses before the global Oligocene-Miocenespread of grasslands. We analyze phytolith assemblages extracted from sediment samples collectedin Paleocene-Eocene Rio Chico Group sites along the San Jorge Basin of Argentine Patagonia to reconstruct the ecosystems early grasses occurred in. The coastal lowland deposits of the LasVioletas up to the base of Las Flores formations of the Rio Chico Group are preliminarily datedbetween 60.01±0.58 Ma to 50.93±0.40 Ma (unpublished information). These deposits producedphytolith assemblages comprising among the oldest occurrences of grass phytoliths in the fossilrecord of South America. Preliminary results of traditional phytolith assemblage analysis andreconstructed canopy structure using epidermal phytolith shape suggest that early grasses in thePaleocene and Eocene of the San Jorge Basin occurred in low abundances in palm and woodydicot-dominated forests of moderately open canopies. These results support the hypothesis that earlygrasses emerged as rare understory components in non-analog shrublands dominated by palms, orforests dominated by woody angiosperms, before expanding to open habitats later in the Cenozoic,where they would become dominant elements of terrestrial landscapes during the mid-late Miocene.Continuing to build the phytolith record in the Cretaceous and early Cenozoic of South America willbe necessary to fully understand the habitats that early grasses occurred in, which will, in turn, shedlight on the timing and possible mechanisms that drove the global expansion of grasses andgrasslands over the last 60 million years. Furthermore, the studied records capture the criticalPaleocene-Eocene transition in southern South America, contributing to our understanding of theimpact of this period of extreme global warming on terrestrial ecosystems in the SouthernHemisphere.